1. Field of the Invention
The invention relates to a method for quality assurance of preferably finger-jointed long timber, produced in series and having a predetermined minimum length, in particular of structural solid wood, as well as to a device for carrying out the method.
2. Description of the Related Art
In order to manufacture from tree trunks a high-quality long timber as used, for instance, as structural timber, thereby ensuring a reasonably uniform quality, the tree trunks are cut or formatted, respectively, to the desired dimension, and, if the blanks thus broken down exhibit defective spots also known as weak points such as loose branches etc., those defective spots or weak points, respectively, are removed and the remaining blank pieces are glued together to form a long timber by means of finger joints on the front side. In this way, glue-laminated timber is produced which is formed from several layers of longitudinally jointed boards glued together and exhibiting finger joints in an offset manner. Laminated squared timber is manufactured from two to three beams glued together longitudinally, which optionally are likewise comprised of sections connected by finger jointings.
A specific problem arises with regard to the processing of strong timber, meaning trees which, at breast height, have a diameter of more than approx. 50 cm. If such strong timber is processed into structural solid wood, it has the advantage of a higher yield. However, the timber properties are very heterogeneous, i.e., the strong timber involves higher selection efforts. In addition, a wet core or a heart shake may cause problems. Strong branches likewise cause poorer mechanical properties. For this reason, long timber manufactured from strong timber can only rarely be cut from a tree trunk in one piece; mostly, it is necessary to cut out weak points and to glue the sections together by finger jointing, as mentioned above, in order to form a long timber.
It is known to carry out this process in a more or less automated fashion, with the wood first passing through a quality sorting plant in which it is examined for moisture, tree-ring density, quality of colour and texture, branches etc., which can either be done visually or by electric resistance measurement (for humidity measurement) or using laser cameras. X-ray or computer-tomography or ultrasonic technology is used for detecting hidden branches. Cutting out the defective spots and sticking together the finger jointing is usually performed in automatically controlled installations.
In order to assure the quality of finger jointings manufactured in such installations, destruction tests are carried out, wherein, in a bend test provided for such a destruction test, no breakage is allowed to occur in the region of the finger jointing.
It has been shown that, if long timber manufactured in this manner is used, unexpected breaks—caused, for example, by compressive breaks e.g. in wind fallen wood, gluing errors, tooth forming errors, internal cracks etc.—may occur despite automated error detecting methods and despite a subsequent intensive visual inspection so that there are attempts to exclude structural solid wood from wooden constructions in which timber has a supporting function. This is disadvantageous especially since, due to this, an inexpensive utilization as a squared timber made of strong timber would no longer be possible, i.e., the strong timber would also have to be processed into multilayered glue-laminated timber or laminated squared timber, wherein hidden or undetected defective spots or weak points, respectively, in individual sections are of less consequence as a result of multilayered gluing.